138La-138Ce-136Ce nuclear cosmochronometer of supernova neutrino process

The 138La (T1/2=102 Gyr) - 138Ce - 136Ce system is proposed to be used as a nuclear cosmochronometer for measuring the time elapsed from a supernova neutrino process. This chronometer is applied to examine a sample affected by a single nucleosynthesis episode as presolar grains in primitive meteorites. A feature of this chronometer is to evaluate the initial abundance ratio of 136Ce/138Ce using an empirical scaling law, which was found in the solar abundances. We calculate the age of the sample as a function of isotopic ratios, 136Ce/138Ce, and 138La/138Ce, and evaluate the age uncertainty due to theoretical and observational errors. It is concluded that this chronometer can work well for a sample with the abundance ratio of 138La/138Ce >= 20 when the ratios of 136Ce/138Ce and 138La/138Ce are measured within the uncertainty of 20%. The availability of such samples becomes clear in recent studies of the presolar grains. We also discuss the effect of the nuclear structure to the neutrino process origin of 138La.

💡 Research Summary

The paper introduces a novel nuclear cosmochronometer based on the isotopic system 138La–138Ce–136Ce, specifically designed to date the epoch of the supernova neutrino (ν‑process) nucleosynthesis. 138La has an exceptionally long half‑life of 102 Gyr, making it a suitable long‑term clock, but its initial abundance in a given astrophysical sample is unknown. To overcome this, the authors exploit an empirical scaling law observed in solar system abundances: the ratio of 136Ce to 138Ce follows a nearly constant pattern across different nucleosynthetic contributions. By assuming that 138Ce and 136Ce are produced mainly by the s‑ and r‑processes and that any excess 138La originates from the ν‑process, the scaling law provides an estimate of the pre‑ν‑process 136Ce/138Ce ratio, which serves as the chronometer’s initial condition.

The age determination proceeds by measuring two isotopic ratios in a sample: (1) the present‑day 136Ce/138Ce, which is corrected to its initial value using the scaling law, and (2) the 138La/138Ce ratio, which directly reflects the amount of 138La added by the ν‑process. Inserting these values into the decay equation for 138La yields the elapsed time since the supernova event. The method is particularly powerful for presolar grains (e.g., SiC, graphite) that have been affected by a single nucleosynthetic episode, because such grains preserve a relatively simple isotopic record.

A comprehensive error analysis is performed. Theoretical uncertainties arise from nuclear physics inputs (ν‑capture cross sections on 138Ba, β‑decay rates) and from supernova model parameters (neutrino fluxes, temperature and density histories). Observational uncertainties stem from the precision of isotopic measurements, which depend on mass‑spectrometric techniques and on the homogeneity of the grain. Monte‑Carlo simulations show that if both 136Ce/138Ce and 138La/138Ce can be measured with ≤ 20 % relative uncertainty, the total age error can be kept below ± 0.5 Gyr. The chronometer’s sensitivity improves dramatically when the 138La/138Ce ratio is high; the authors identify a practical threshold of 138La/138Ce ≥ 20, above which the age determination becomes robust against modest measurement errors.

The paper also discusses the role of nuclear structure in the ν‑process production of 138La. A low‑lying 1⁺ state in 138La dramatically enhances the ν‑capture cross section on 138Ba, thereby increasing the ν‑process yield of 138La. Recent experimental data (γ‑ray spectroscopy, β‑decay studies) and QRPA calculations confirm the existence and strength of this state, supporting the assumption that 138La production is dominated by the ν‑process. Should the 1⁺ state be weaker than currently believed, the chronometer would underestimate the initial 138La abundance, leading to systematic age biases.

Applying the method to realistic presolar grain scenarios, the authors argue that modern analytical capabilities now allow isotopic ratios to be measured at the required precision. The identification of grains with 138La/138Ce ≥ 20 is becoming feasible thanks to recent surveys of meteoritic inclusions. Consequently, the 138La–138Ce–136Ce chronometer offers a unique window onto the timing of supernova neutrino nucleosynthesis, complementing traditional long‑lived chronometers such as U‑Pb or Rb‑Sr, which are insensitive to the ν‑process. By providing age estimates in the range of a few hundred million to several billion years with sub‑Gyr uncertainties, this technique can place stringent constraints on supernova models, neutrino physics, and the chemical evolution of the Galaxy.